PROTECTED IMAGE

Abstract: A binary image comprising a plurality of first areas, at which a porous or particulate image-forming substance is adhered to a substrate, and a plurality of second areas, at which the substrate is free from the image-forming substance, is protected by laminating thereto a laminating sheet comprising a barrier layer (34), a durable layer (36) and a support layer (40) with the barrier layer facing the image (10b), so that the barrier and durable layers adhere to both the first and second areas of the image. The support layer is then displaced away from the image such that the barrier and durable layers remain attached to the image. Both the barrier and durable layers are substantially transparent and the barrier layer comprises a polymeric organic material substantially impervious to the passage of hexane, isopropanol or water.

Description: PROTECTED IMAGE This invention relates to a protected image and to a process for the production of such an image. International Patent Application No. PCT US87/03249 (Publication No. WO 88/04237) describes a thermal imaging medium and a process for forming an image in which a layer of a porous or paniculate image-forming substance (preferably, a layer of carbon black) is deposited on a heat-activatable image-forming surface of a first sheet-like or web material (hereinafter the "first sheet element"), the layer having a cohesive strength greater than its adhesive strength to the first sheet-like element. Portions of this thermal imaging medium are then exposed to brief and intense radiation (for example, by laser scanning), to firmly attach exposed portions of the image-forming substance to the first sheet element. Finally, those portions of the image-forming substance not exposed to the radiation (and thus not firmly attached to the first sheet element) are removed, thus forming a binary image comprising a plurality of first areas, where the image-forming substance is adhered to the first sheet-like element, and a plurality of second areas, where the first sheet-like element is free from the image- forming substance. Hereinafter, this type of image will be called a "differential adhesion" image. In a preferred embodiment of the imaging medium described in this International Application, the image-forming substance is covered with a second laminated sheet-like element so that the image-forming substance is confined between the first element and this second element. After imaging and separation of the unexposed portions of the image-forming substance (with the second element) from the first element, a pair of images is obtained. A first image comprises exposed portions of image-forming substance more firmly attached to the first element by heat activation of the heat-activatable image- forming surface. A second image comprises non-exposed portions of the image-forming substance carried or transferred to the second sheet element. The respective images obtained by separating the sheets of an exposed thermal imaging medium having an image-forming substance confined therebetween may exhibit substantially different characteristics. Apart from the imagewise complementary nature of these images and the relation that each may bear as a "positive" or "negative" of an original, the respective images may differ in character. Differences may depend upon the properties of the image-forming substance, on the presence of additional layer(s) in the medium, and upon the manner in which such layers fail adhesively or cohesively upon separation of the sheets. Either of the pair of images may, for reasons of informational content, aesthetics or otherwise, be desirably considered the principal image, and all of the following discussion is applicable to both types of image. The image-forming process described in this International Application can produce high quality, high resolution images. However, the images produced by this process may suffer from low durability because, in the finished image, the porous or particulate image-forming substance, which is typically carbon black admixed with a binder, lies exposed on the surface of the image, and may be smeared, damaged or removed by, for example, fingers or other skin surfaces (especially if moist), solvents or friction during manual or other handling of the image. International Application No. PCT US91/08345 (published as WO 92/09930 on June 11, 1992) describes a process for protecting a binary image, such as that produced by the aforementioned International Application No. PCT/US87/03249, having a plurality of first areas, at which a porous or particulate image-forming substance is adhered to a substrate, and a plurality of second areas, at which the substrate is free from the image-forming substance. This protecting process is carried out by laminating to the image a laminating sheet comprising a durable layer and a support layer, with the durable layer facing the image, so that the durable layer adheres to both the first and second areas of the image. The support layer is then displaced away from the image such that the image remains covered with a durable layer which: a) is substantially transparent; b) has an abrasion resistance of at least 10 cycles of a 10 Newton force as measured by an Erichsen Scar Resistance Tester (referred to as an Erikson Abrasion Meter in the aforementioned International Application No. PCT/US91/08345) and a critical load value of at least 100 grams as measured by ANSI PHI.37- 1983; and c) is not removed from the image by contact with adhesive tape having an adhesion to steel of 33 grams per millimeter as measured by ASTM D-3330. The preferred durable layers for use in this process are acrylic polymers, and the process provides the binary images with protection adequate for many fields in which such images are used. However, binary images having the specific durable layers mentioned in the aforementioned International Application No. PCT/US91/08345 are not entirely satisfactory for use as copying media in the graphic arts industry. In this industry, it is common practice to position images securely in layouts with a strong adhesive tape (hereinafter called "graphic arts tape," and also referred to in the industry as "ruby tape"; one major brand is sold commercially as "Red Lithographers tape #616" by Minnesota Mining and Manufacturing Corporation, St. Paul, Minnesota, 55144-1000, United States of America), and it is frequently necessary to secure an image with such tape and later to peel the tape from the image, and then to repeat this process several times. Also, in this industry images are subject to multiple washings with isopropanol and other solvents to ensure the high degree of cleanliness needed in images used for further copying. It has been found that under the extreme stresses caused by such repeated applications of graphic arts tape and repeated washings, the durable layers mentioned in the aforementioned International Application No. PCT US91/08345 may not adhere adequately to the underlying image. Accordingly, there is a need for protection of such binary images so as to render the protected image durable, transparent and abrasion- resistant, and permits repeated applications of graphic arts tape, and repeated solvent washings of the protected image, without risk of separation of the durable layer from the binary image. European Patent Application No. 94107010.4 describes a process for protecting a binary image which is generally similar to that described in the aforementioned International Application No. PCT/US91/08345, but in which the durable layer comprises a polymeric organic material having incorporated therein a siloxane, the siloxane being incorporated into the polymeric material so that it is not removed therefrom by hexane, isopropanol or water. The presence of the siloxane in the durable layer allows repeated applications of graphic arts tape to the protected image without damage to the durable layer and the image, and also allows repeated solvent washings of the protected image. However, it has been found that protected binary images produced in accordance with this European application can still be damaged by prolonged exposure to solvents such as those used in the graphic arts industry. If the protected images are exposed to solvents for a long period and/or the exposed section of the image is rubbed repeatedly, in some cases disruption of the image occurs, i.e., the colored image-forming substance disappears from the rubbed portion of the image. It has now been found that the aforementioned image disruption is due, at least in part, to solvent penetrating the durable layer of the protected image and rendering the image-forming substance semi-solid, so that it can move between the durable layer and the substrate. Such penetration of solvent through the durable layer does not significantly affect the durability of the layer. However, despite extensive experimentation, it has not been possible to find a material for the durable layer which will permit repeated applications of graphic arts tape thereto without damage to the image, and which will completely prevent diffusion of solvent through the durable layer. Moreover, attempts to modify the durable layer to increase its resistance to solvent diffusion therethrough often adversely affect the ability of the durable layer to resist repeated applications of graphic arts tape. We have determined that the resistance of differential adhesion images to damage by solvents can be increased by including, between the durable layer and the image to be protected, a barrier layer comprising a polymeric organic material substantially impervious to the passage of hexane, isopropanol and water therethrough. Accordingly, this invention provides a process, generally similar to those described in the aforementioned International Application No. PCT/US91/08345 for protecting a binary image, this binary image comprising a plurality of first areas, at which a porous or particulate image-forming substance is adhered to a substrate, and a plurality of second areas, at which the substrate is free from the image-forming substance. The process comprises: providing a laminating sheet comprising a durable layer and a support layer, the durable layer being substantially transparent ; laminating the laminating sheet to the binary image so that the durable layer adheres to both the first and second areas of the image; and separating the support layer from the image such that the durable layer remain attached to the image, thereby covering the image with a durable layer. The present process is characterized in that the laminating sheet further comprises a barrier layer disposed on the opposed side of the durable layer from the support layer, this barrier layer being substantially transparent and comprising a polymeric organic material substantially impervious to the passage of hexane, isopropanol and water therethrough, such that, following the lamination, the barrier layer adheres to both the first and second areas of the image, and after the separation of the support layer from the image the barrier layer remains attached to the image. This invention also provides a protected binary image, the image comprising a plurality of first areas at which a porous or particulate image-forming substance is adhered to a substrate and a plurality of second areas at which the substrate is free from the image-forming substance, and a substantially transparent durable layer covering the image and adhered to both the first and second areas of the image. The protected image of this invention is characterized by a barrier layer covering the image and disposed between the barrier layer and the image, the barrier layer being substantially transparent and adhered to both the first and second areas of the image, the barrier layer comprising a polymeric organic material substantially impervious to the passage of hexane, isopropanol and water therethrough. Figure 1 of the accompanying drawings shows in section a thermal imaging medium of the type described in the aforementioned European Application Serial No. 94107010.4; Figure 2 shows a section, similar to that of Figure 1 through the medium as the first and second elements thereof are being separated to form a pair of complementary binary images; Figure 3 shows a section through one of the binary images formed in Figure 2 and a laminating sheet useful in the process of the present invention; Figure 4 shows in section the image and laminating sheet shown in Figure 3 laminated together; Figure 5 shows in section the image and laminating sheet shown in Figures 3 and 4 as the support layer is being separated from the image; Figure 6 shows in section the protected image produced after complete removal of the support layer; and Figure 7 shows a schematic side elevation of an apparatus useful for carrying out the process of the invention. In the present process, the binary image is covered with a two-layer covering, this covering comprising a barrier layer covering the image and a durable layer also covering the image and disposed on the face of the barrier layer remote from the image. Both the barrier and durable layers are substantially transparent, so that the image can be viewed through these two layers, and the barrier and durable layers adhere to both the first and second areas of the image. The barrier layer comprises a polymeric organic material substantially impervious to the passage of hexane, isopropanol and water therethrough. The barrier layer prevents solvents (such as those typically contained in graphic arts cleaning solutions) which may penetrate the durable layer from entering the layer of image-forming substance and inducing the changes in this layer of image- forming substance which may lead to image disruption. The provision of the barrier layer not only improves the solvent resistance of the protected image but also simplifies the problem of finding a durable layer which will resist repeated applications of graphic arts tape. As already noted, attempts to modify the durable layer to increase its resistance to solvent diffusion therethrough often adversely affect the ability of the durable layer to resist repeated applications of graphic arts tape, and thus providing sufficient solvent resistance often compromises the ability of the durable layer to resist repeated applications of graphic arts tape. When a barrier layer is provided in accordance with the present invention, the composition of the durable layer can be varied to provide maximum resistance to repeated applications of graphic arts tape without worrying about solvent resistance, while the composition of the barrier layer can be optimized for maximum solvent resistance. The barrier layer may be formed from any polymeric organic material which is substantially impervious to the passage of hexane, isopropanol and water, provided of course that the barrier layer can be made to adhere sufficiently to the durable layer and to the image to prevent damage to the protected image due to mechanical stresses imposed upon the protected image during its intended use. Because of their high resistance to solvent penetration, preferred materials for use in the barrier layer are those comprising polymerized repeating vinylidene chloride units. Desirably, the barrier layer also comprises copolymerized repeating units from an ethylenically unsaturated monomer copolymerizable with vinylidene chloride, this ethylenically unsaturated monomer preferably being an acrylate or methacrylate. A preferred copolymer of this type is that sold commercially as Daran SL-158 by Hampshire Chemical Corporation, of 55 Hayden Road, Lexington MA 02173, United States of America; this material is stated by the manufacturer to be a copolymer of vinylidene chloride and methyl acrylate. Polyurethanes may also be used in the barrier layer; preferred polyurethanes for this purpose are water-dispersible polyurethanes based upon aliphatic polyisocyanates. A specific polyurethane which has been found to give good results in the present process may be obtained by adding Bayhydrol 116 to about 3.5 times its own volume of water, and using the resultant polyurethane dispersion directly as the coating fluid. (Bayhydrol 116 is a water-reducible blocked polyisocyanate, sold commercially by Miles Industrial Chemical Division, Mobay Road, Pittsburgh PA 15205-9741, United States of America, and is stated by its seller to be an aliphatic polyisocyanate based on hexamethylene diisocyanate.) The barrier layer need only be thick enough to give effective protection against penetration of solvents into the image, and in view of the desirability of keeping the total thickness of the barrier and durable layers small (for reasons discussed in detail below), it is preferred to keep the barrier layer thickness in the range of 0.5 to 5 μm. Although the optimum thickness of the barrier layer will of course vary with the composition of the barrier and other layers, and the expected conditions of use of the protected image, in general barrier layers about 1 μm thick have been found to give satisfactory results. Since the purpose of the durable layer in the present invention is to protect the image from damage or abrasion, and to resist the effects of application of graphic arts tape, the durable layer is preferably derived from a monomer which forms a durable and substantially transparent homopolymer, for example homo- and copolymers of acrylates and methacrylates, especially poly(methyl methacrylate). Desirably, the durable layer is as described in the aforementioned European Application No. 94107010.4, and comprises a siloxane, the siloxane being incorporated into a polymeric material so that it is not removed therefrom by hexane, isopropanol or water. Incorporation of the siloxane into the durable layer to meet this requirement can be effected in various ways. For example, the durable layer may be formed by providing a mixture of an organic polymer, a polymerizable monomer or oligomer of a siloxane, and a polymerization initiator, and subjecting this mixture to conditions effective to activate the polymerization initiator, thereby causing polymerization of the siloxane monomer or oligomer, and formation of the polymeric organic material containing the siloxane. It is believed that this method of forming the polymeric organic material typically produces a semi-interpenetrating network with a network of polymerized siloxane extending through the network formed by the organic polymer. The polymerization initiator may be a thermal initiator (for example, a peroxide or 2,2'- azobis(2-rnethylpropionitrile) (usually known as AIBN)), which is activated by heating the layer of the mixture on the support layer, or the initiator may be a photoinitiator (for example 2,2-dimethoxy-2-phenylacetophenone, available as Irgacure 651 from Ciba- Geigy Corporation, 7 Skyline Drive, Hawthorne, New York 10532-2188, United States of America, which is activated by exposure to ultra-violet radiation). Desirably, in some cases, the mixture includes a cross-linking agent; preferred cross-linking agents for use with the preferred siloxanes discussed below are pentaerythritol triacrylate (PETA) and trimethylolpropane triacrylate (TMPTA). Alternatively, the organic polymeric material may be a graft copolymer of a siloxane and an organic monomer. Techniques for preparing such graft copolymers in solution are well known to those skilled in the art of polymer synthesis. Examples 5 and 6 of the aforementioned European Application No. 94107010.4 illustrate a specialized technique for synthesis of such graft copolymers in aqueous media; in this technique, a siloxane oligomer having one ethylenically-unsaturated end group is copolymerized with an ethylenically-unsaturated organic monomer to form a graft copolymer having siloxane side-chains. Another preferred siloxane-containing polymeric organic material for use in the present process is prepared by copolymerizing a siloxane monomer or oligomer with an organic monomer or oligomer which has been functionalized with vinyl ether groups. A variety of such vinyl ether functionalized monomers and oligomers are available commercially including, for example, VEctomer 2010, a vinyl ether functionalized aromatic urethane oligomer and VEctomer 4010, a divinyl ether functionalized aromatic ester monomer, both sold by Allied Signal Corporation, Morristown, New Jersey 07962, United States of America, and Rapi-Cure (Registered Trade Mark) CHVE, a divinyl ether functionalized cyclohexane, sold by GAF Corporation, Wayne, New Jersey 07470, United States of America. Typically, the mixture of the functionalized monomer or oligomer and the siloxane is polymerized by adding a sensitizer, for example a sulfonium salt, and exposing the mixture to ultra-violet radiation. The optimum portion of siloxane in the durable layer is best determined empirically. Although larger proportions of siloxane may sometimes be desirable, typically, good results can be obtained using not more than about 10 percent by weight of siloxane in the durable layer, and in many cases not more than about 5 weight percent. Especially when the durable layer is formed by polymerizing the siloxane in the presence of a pre-formed organic polymer, inclusion of excess siloxane may reduce the durability of the durable layer by lowering the glass transition temperature of the cured polymeric durable layer, and may allow phase separation of the organic material/siloxane mixture before or after curing. In general, it is preferred that the barrier and durable layers on the image not have a total thickness greater than 30 μm, since thicker barrier and durable layers may sometimes cause optical problems in viewing the image due to internal reflections and/or refraction effects within or between the barrier and durable layers, and the thicker these layers, the more they absorb. Also, when a protected image is used to expose a radiation-sensitive material, the durable layer is placed in contact with the radiation-sensitive material. Consequently, the total thickness of the barrier and durable layers affects the resolution achievable in the final image in the radiation-sensitive material. To prevent undesirable loss of resolution, it is in general desirable that the barrier and durable layers formed on the image have a total thickness not greater than 10 μm, and preferably in the range of from 0.5 to 6 μm, since layers of these thicknesses normally do not cause optical problems in viewing the image, and permit exposure of radiation-sensitive materials through the protected image without adversely affecting the resolution of the image produced. To produce a sufficiently thin durable layer smooth enough to prevent undesirable optical effects when the protected image is used to expose a radiation-sensitive material, it is convenient to form the durable layer in situ by forming the necessary polymerizable mixture, spreading a layer of this mixture upon the support layer, and subjecting the layer of the mixture to conditions effective to cause polymerization to form the final durable layer, provided of course that the polymerization technique used is one which can be practiced under these conditions. As noted in the aforementioned European Application No. 94107010.4, a differential adhesion image typically extends close to the periphery of the substrate, since for practical reasons it is desirable to coat the various layers of the differential adhesion imaging medium, including the porous or particulate image-forming substance, on large webs and then to divide these webs into the smaller sheets required for individual images. To protect a differential adhesion image extending close to the periphery of the substrate, it is necessary that the barrier and durable layers also extend to this periphery; on the other hand, both for aesthetic reasons and for ease of handling, surplus barrier and durable layer should not extend beyond the periphery of the substrate, and the process for applying the protective layer should not require elaborate procedures for registering the barrier and durable layers with the image. Accordingly, in a preferred form of the present process, the laminating sheet is laminated to the binary image such that at least one portion of the laminating sheet extends beyond the periphery of the substrate, and the support layer is separated from the image such that, in this portion or portions of the laminating sheet, the barrier layer and the durable layer remain attached to the support layer so that the barrier layer and the durable layer break substantially along the periphery of the substrate. The support layer of the laminating sheet may be formed from any material which can withstand the conditions which are required to laminate the laminating sheet to the image and which is sufficiently coherent and adherent to the durable layer to permit displacement of the support layer away from the image after lamination, with removal of those portions of the barrier and durable layers which extend beyond the periphery of the substrate. Typically, the support layer is a plastic film, and polyester (preferably poly(ethylene terephthalate)) films are preferred. A film with a thickness in the range of 0.5 to 2 mil (13 to 51 μm) has been found satisfactory. If desired, the support layer may be treated with a subcoat or other surface treatment, such as will be well known to those skilled in the coating art, to control its surface characteristics, for example to increase or decrease the adhesion of the durable layer or other layers (see below) to the support layer. The laminating sheet may comprise additional layers besides the barrier layer, durable layer and support layer. For example, the laminating sheet may comprise a release layer interposed between the durable layer and the support layer, this release layer being such that, in the areas where the barrier and durable layers remain attached to the image, separation of the durable layer from the support layer occurs by failure within or on one surface of the release layer. The release layer is preferably formed from a wax, or from a silicone. In some cases part or all of the release layer may remain on the surface of the durable coating after the support layer has been removed, and if a radiation-sensitive material is to be exposed through the protected image, care must be taken to ensure that any remaining release layer on the protected image does not interfere with such exposure. The laminating sheet may also comprise an adhesive layer disposed on the surface of the barrier layer remote from the support layer so that, during the lamination, the barrier and durable layers are adhered to the image by the adhesive layer. In general, the use of an adhesive layer is desirable to achieve strong adhesion between the barrier layer and the image, and/or to lower the temperature needed for lamination. Various differing types of adhesive may be used to form the adhesive layer; for example, the adhesive layer might be formed from a thermoplastic (hot melt) adhesive and the lamination effected by heating the adhesive layer above its glass transition temperature. A preferred hot melt adhesive for this purpose is an ethylene/vinyl acetate copolymer, for example that sold as Morton Adcote 9636/37 hot melt adhesive, by Morton International, Inc., 3334 West Wacker Drive, Chicago, LL 60606, United States of America. Alternatively, the adhesive may be an ultraviolet curable adhesive (in which case the lamination is performed with the uncured adhesive, after which the adhesive is exposed to ultraviolet radiation, so curing the adhesive layer), or a pressure sensitive adhesive, typically one having an adhesion to steel of 22 to 190 grams per millimeter (in which case the lamination is effected simply by pressure). The durable layer formed on the image should desirably adhere sufficiently to the image that it is not removed therefrom by repeated contact with graphic arts tape before or after application of solvents used in the graphics art industry for cleaning films. Desirably the durable layer provided on the image by the present processes has an abrasion resistance of at least 10 cycles of a 10 Newton force as measured by an Erichsen Scar Resistance Tester, and is not removed from the image by adhesive tape having an adhesion to steel of 33 grams per millimeter, as measured by ASTM D-3330. The various layers of the laminating sheet used in the present process may be formed by conventional techniques which will be familiar to those skilled in the laminating art. Thus, the barrier and durable layers (and the release and adhesive layers, when present) are typically deposited in order upon the support layer, deposition being effected by coating from aqueous or organic solvents, or in some cases by extrusion of the layer on to the support. If the present process is to be used to produce a protected image intended to be viewed in reflection, the substrate of the image may be opaque, and may be formed from paper or a similar material. However, typically the substrate of the image will be essentially transparent, and the substrate will be a plastic web having a thickness of from 1 to 1000 μm, and preferably 25 to 250 μm. As is well known to those skilled in the imaging art, the substrate may carry one or more sub-coats or be subjected to surface treatment to improve the adhesion of the image-forming substance to the substrate. Materials suitable for use as the substrate include polystyrene, polyester, polyethylene, polypropylene, copolymers of styrene and acrylonitrile, poly(vinyl chloride), polycarbonate and poly(vinylidene chloride). An especially preferred web material from the standpoints of durability, dimensional stability and handling characteristics is poly(ethylene terephthalate), commercially available, for example, under the tradename Mylar, of E. I. du Pont de Nemours & Co., Wilmington, Delaware, United States of America, or under the tradename Kodel, of Eastman Kodak Company, Rochester, New York, United States of America. The image-forming substance typically comprises a porous or particulate colorant material admixed with a binder, the preferred colorant material being carbon black, although other optically dense colorants, for example graphite, phthalocyanine pigments and other colored pigments may be used. The binder may be, for example, gelatin, poly(vinyl alcohol), hydroxyethylcellulose, gum arabic, methylcellulose, polyvinylpyrrolidone or polyethyloxazoline. The images protected by the process of the present invention may be of various types. For example, the present process could be used for protecting radiographs, CAT scans, ultrasonograms and similar medical images. Often, the medical personnel using such images will need to view them on conventional lightboxes, to which the images will be fixed with heavy metal clips. Accordingly, in this application it is important that the durable layer withstand repeated affixation to a lightbox by means of such clips. However, as already mentioned, the present invention is primarily intended for use in the graphics arts industry in the production of films (including separation, imagesetter, contact, duplicating, camera and other films) and of pre-press proofs. In the printing industry, it is conventional practice to form images of originals on separation imaging film (a single image for monochrome printing, or a series of color separations for color printing) and then to prepare a printing plate, or additional intermediate films or proofs, by contact exposing a radiation-sensitive material through the separation imaging film. Conventional practices in the printing industry make stringent demands upon separation film images. The image must, of course, have high optical clarity so that exposure of a printing plate can be effected through the image. The need for exposure of the radiation-sensitive material through the film also requires that the thickness of the layers in the film be limited. The separation film image must have good abrasion resistance against general handing and cleaning so that it can withstand being pressed against the radiation-sensitive material, removed therefrom, stored for an extended period and then reused for making another printing plate, or additional intermediate films or proofs. The separation film image must also have non-blocking properties. When the protected image of the present invention is to be used for exposing a radiation-sensitive material, the barrier and durable coatings over the image must transmit the radiation used to expose the radiation-sensitive material; in particular, in many commercial applications, these coatings and the substrate should transmit ultraviolet and visible radiation in the wavelength range of 300 to 460 run. When a protected image of this invention is used to expose a radiation- sensitive material, the durable layer is normally placed in contact with the radiation-sensitive material. Consequently, the total thickness of the barrier and durable layers affects the resolution achievable in the final image in the radiation-sensitive material. As already mentioned, to prevent undesirable loss of resolution, it is in general desirable that the barrier and durable coatings formed on the image have a total thickness not greater than 30 μm, desirably not greater than 12 μm, and preferably in the range of from 0.5 to 10 μm, since barrier and durable coatings of these thicknesses normally do not cause optical problems in viewing the image, and permit exposure of radiation-sensitive materials through the protected image without adversely affecting the resolution of the image produced. It should be noted that some plastics normally regarded as durable when in thick layers are insufficiently durable in 2 to 6 μm layers, and acrylic polymers, for example poly(methyl methacrylate), polystyrenes and polyurethanes are the preferred materials for forming the durable layer. To allow the protected image to be exposed using the vacuum frames conventional in the printing industry, desirably the barrier and durable layers provide coatings which can sustain a vacuum drawdown of 660 mm Hg for five minutes without the appearance of Newton's rings. It is also desirable that the durable coating produced survive intimate contact by vacuum drawdown for five minutes with other films and plates without blocking or other damage to the film or protected image. To avoid air being trapped between the protected image and the radiation-sensitive material, it is desirable that the durable coating produced have a matte, slightly roughened surface, since such a matte surface allows for escape of air from between the durable coating and the radiation-sensitive material with which it is in contact, thus preventing the formation of Newton's rings and other undesirable interference phenomena caused by trapped air. It has been found that the texture of the surface of the support layer in contact with the durable layer affects the texture of the durable coating produced, and accordingly it is desirable that this surface be matte. In the production of printing plates, it is highly desirable that the operator be able to distinguish visually between the two sides of the protected image in order to avoid accidental inversion of the protected image, with consequent lateral inversion of the image formed on the printing plate. Accordingly, it is preferred that the durable layer formed on the image have a gloss number in the range of from 50 to 100 at a 60° angle, desirably 60 to 80 at this angle. A similar gloss number is desirable for protected medical images to prevent unfortunate accidents caused by accidental lateral inversion of the image of a patient being treated. In Figure 1, there is shown a preferred laminar imaging medium (generally designated 10) of the present invention suited to production of a pair of high resolution images, shown in Figure 2 as images 10a and 10b in a partial state of separation. Thermal imaging medium 10 includes a first element in the form of a first sheet-like or web material 12 (comprising sheet material 12a, stress-absorbing layer 12b and heat-activatable zone or layer 12c) having superposed thereon, and in order, porous or particulate image-forming layer 14, release layer 16, first adhesive layer 18, second, hardenable polymeric adhesive layer 20 and second sheet-like or web material 22. Upon exposure of the medium 10 to infra-red radiation, exposed portions of image-forming layer 14 are more firmly attached to web material 12, so that, upon separation of the respective sheet-like materials, as shown in Figure 2, a pair of images, 10a and 10b, is provided. The nature of certain of the layers of preferred thermal imaging medium material 10 and their properties are importantly related to the manner in which the respective images are formed and partitioned from the medium after exposure. The various layers of medium material 10 are described in detail below. Web material 12 comprises a transparent material through which imaging medium 10 can be exposed to radiation. Web material 12 can comprise any of a variety of sheet-like materials, although polymeric sheet materials will be especially preferred. Among preferred sheet materials are polystyrene, poly(ethylene terephthalate), polyethylene, polypropylene, poly(vinyl chloride), polycarbonate, poly(vinylidene chloride), cellulose acetate, cellulose acetate butyrate and copolymeric materials such as the copolymers of styrene, butadiene and acrylonitrile, including poly(styrene-co- acrylonitrile). The stress-absorbing layer 12b is as described in U.S. Patent No. 5,200,297 and the corresponding International Patent Application No. PCT/US91/08604 (Publication No. WO 92/09443), and comprises a polymeric layer capable of absorbing physical stresses applied to the imaging medium 10. The stress-absorbing layer 12b provides added protection against delamination of the medium 10 when rigorous physical stresses are applied thereto, and is desirably formed from a compressible or elongatable polyurethane. The stress-absorbing layer 12b is optional and may sometimes be omitted, depending upon the second adhesive layer 20 used and the stresses to which the medium 10 will be subjected. Heat-activatable zone or layer 12c provides an essential function in the imaging of medium 10 and comprises a polymeric material which is heat activatable upon subjection of the medium to brief and intense radiation, so that, upon rapid cooling, exposed portions of the surface zone or layer 12c are firmly attached to porous or particulate image-forming layer 14. If desired, when the stress-absorbing layer 12b is omitted, surface zone 12c can be a surface portion or region of web material 12, in which case, layers 12a and 12c will be of the same or similar chemical composition. In general, it is preferred that layer 12c comprise a discrete polymeric surface layer on sheet material 12a or stress-absorbing layer 12b. Layer 12c desirably comprises a polymeric material having a softening temperature lower than that of sheet material 12a, so that exposed portions of image-forming layer 14 can be firmly attached to web material 12. A variety of polymeric materials can be used for this purpose, including polystyrene, poly(styrene-co-acrylonitrile), poly(vinyl butyrate), poly(methyl methacrylate), polyethylene and poly(vinyl chloride). The employment of a thin heat-activatable layer 12c on a substantially thicker and durable sheet material 12a permits desired handling of the web material and desired imaging efficiency. The use of a thin heat-activatable layer 12c concentrates heat energy at or near the interface between layers 12c and image-forming layer 14 and permits optimal imaging effects and reduced energy requirements. It will be appreciated that the sensitivity of layer 12c to heat activation (or softening) and attachment or adhesion to layer 14 will depend upon the nature and thermal characteristics of layer 12c and upon its thickness. Stress-absorbing layer 12b can be provided on sheet material 12a by the methods described in the aforementioned U.S. Patent No. 5,200,297 and International Patent Application No. PCT/US91/08604. Heat-activatable layer 12c can be provided by resort to known coating methods. For example, a layer of poly(styrene-co- acrylonitrile) can be applied to a web of poly(ethylene terephthalate) by coating from an organic solvent such as methylene chloride. The desired handling properties of web material 12 will be influenced mainly by the nature of sheet material 12a itself, since layers 12b and 12c will be coated thereon as thin layers. The thickness of web material 12 will depend upon the desired handling characteristics of medium 10 during manufacture, imaging and any post-imaging steps. Thickness will also be dictated in part by the intended use of the image to be carried thereon and by exposure conditions, such as the wavelength and power of the exposing source. Typically, web material 12 will vary in thickness from 0.5 to 7 mil (13 to 178 μm). Good results are obtained using, for example, a sheet material 12a having a thickness of 1.5 to 1.75 mils (38 to 44 μm). Stress-absorbing layer 12b will typically have a thickness in the range of 1 to 4 μm, while layer 12c will typically be a layer of poly(styrene-co-acrylonitrile) having a thickness of 0.1 to 5 μm. Heat-activatable layer 12c can include additives or agents providing known beneficial properties. Adhesiveness-imparting agents, plasticizers, adhesion- reducing agents, or other agents can be used. Such agents can be used, for example, to control the adhesion between layers 12c and 14, so that undesirable separation at the interface is minimized during the manufacture of laminar medium 10 or its use in a thermal imaging method or apparatus. Such control also permits the medium, after imaging and separation of sheet-like web materials 12 and 22, to be partitioned in the manner shown in Figure 2. Image-forming layer 14 comprises an image-forming substance deposited on to heat-activatable zone or layer 12c as a porous or particulate layer or coating. Layer 14, also called a colorant/binder layer, can be formed from a colorant material dispersed in a suitable binder, the colorant being a pigment or dye of any desired color, and preferably being substantially inert to the elevated temperatures required for thermal imaging of medium 10. Carbon black is a particularly advantageous and preferred pigment material. Preferably, the carbon black material will comprise particles having an average diameter of 0.01 to 10 μm. Although the description herein will refer principally to carbon black, other optically dense substances, such as graphite, phthalocyanine pigments and other colored pigments can be used. If desired, substances which change their optical density upon subjection to temperatures as herein described can also be employed. The binder for the image-forming substance or layer 14 provides a matrix to form the porous or particulate substance into a cohesive layer. This binder also serves to adhere layer 14 to heat-activatable zone or layer 12c. In general, it will be desired that image-forming layer 14 be adhered to surface zone or layer 12c sufficiently to prevent accidental dislocation either during the manufacture of medium 10 or during its use. Layer 14 should, however, be separable (in non-exposed regions) from zone or layer 12c, after imaging and separation of webs 12 and 22, so that partitioning of layer 14 can be accomplished in the manner shown in Figure 2. Image-forming layer 14 can be conveniently deposited on to surface zone or layer 12c, using known coating methods. According to one embodiment, and for ease in coating layer 14 on to zone or layer 12c, carbon black particles are initially suspended in an inert liquid vehicle, with a binder or dispersant, and the resulting suspension or dispersion is uniformly spread over heat-activatable zone or layer 12c. On drying, layer 14 is adhered as a uniform image-forming layer on the surface zone or layer 12c. It will be appreciated that the spreading characteristics of the suspension can be improved by including a surfactant, such as ammonium perfluoroalkyl sulfonate, non-ionic ethoxylate or the like. Other substances, such as emulsifiers, can be used or added to improve the uniformity of distribution of the carbon black in either its suspended or its spread and dry state. Layer 14 can vary in thickness and typically will have a thickness of 0.1 to 10 μm. In general, it is preferred, for high image resolution, that a thin layer 14 be employed. Layer 14 should, however, be of sufficient thickness to provide desired and predetermined optical density in the images prepared from imaging medium 10. Suitable binder materials for image-forming layer 14 include gelatin, poly(vinyl alcohol), hydroxyethyl cellulose, gum arabic, methyl cellulose, polyvinylpyrrolidone, polyethyloxazoline, polystyrene latex and poly(styrene-co-maleic anhydride). The ratio of pigment (e.g., carbon black) to binder can be in the range of from 40: 1 to 1:2 on a weight basis. Preferable, the ratio of pigment to binder will be from 4:1 to 10:1. A preferred binder material for a carbon black pigment material is poly(vinyl alcohol). If desired, additional additives or agents can be incorporated into image- forming layer 14. Thus, submicroscopic particles, such as chitin, polytetrafluoroethylene particles and/or polyamide can be added to colorant/binder layer 14 to improve abrasion resistance. Such particles can be present, for example, in amounts of from 1 :2 to 1 :20, particles to layer solids, by weight. Porous or particulate image-forming layer 14 can comprise a pigment or other colorant material such as carbon black which is absorptive of exposing radiation, and is known in the thermographic imaging field as a radiation-absorbing pigment. Since secure bonding or joining is desired at the interface between layer 14 and heat- activatable zone or layer 12c, it may sometimes be preferred that a radiation-absorbing substance be incorporated into either or both of image-forming layer 14 and heat- activatable zone or layer 12c. Suitable radiation-absorbing substances in layers 14 and/or 12c, for converting radiation into heat, include carbon black, graphite or finely divided pigments such as the sulfides or oxides of silver, bismuth or nickel. Dyes such as the azo dyes, xanthene dyes, phthalocyanine dyes or anthraquinone dyes can also be employed for this purpose. Especially preferred are materials which absorb efficiently at the particular wavelength of the exposing radiation. Infrared dyes which absorb in the infrared-emitting regions of lasers which are desirably used for thermal imaging are especially preferred. Suitable examples of infrared-absorbing dyes for this purpose include the alkylpyrylium- squarylium dyes, disclosed in U.S. Patent No. 4,508,811, and including l,3-bis[(2,6-di-t- butyl-4H-thiopyran-4-ylidene)methyl]-2,4-dihydroxy-dihydroxide-cyclobutene diylium- bisjinner salt}. Other suitable infrared-absorbing dyes include those described in U.S. Patent No. 5,231,190 (and in the corresponding European Application No. 92107574.3, Publication No. 516,985); in International Application No. PCT/US91/08695, Publication No. WO 92/09661; and in U.S. Patents Nos. 5,227,498 and 5,227,499. For the production of images of high resolution, it is essential that image- forming layer 14 comprise materials that permit fracture through the thickness of the layer and substantially orthogonal to the interface between surface zone or layer 12c and image-forming layer 14, i.e., substantially along the direction of arrows 24, 24', 26, and 26', shown in Figure 2. It will be appreciated that, in order for images 10a and 10b to be partitioned in the manner shown in Figure 2, image-forming layer 14 will be orthogonally fracturable as described above and will have a degree of cohesivity greater than its adhesivity for heat-activatable zone or layer 12c. Thus, on separation of webs 12 and 22 after imaging, layer 14 will separate in non-exposed areas from heat- activatable layer 12c and remain in exposed areas as porous or particulate portions 14a on web 12. The release layer 16 shown in Figure 1 is included in thermal imaging medium 10 to facilitate separation of images 10a and 10b according to the mode shown in Figure 2. As described above, regions of medium 10 subjected to radiation become more firmly secured to heat-activatable zone or layer 12c because of the heat activation of the layer by the exposing radiation. Non-exposed regions of layer 14 remain only weakly adhered to heat-activatable zone or layer 12c and are carried along with sheet 22 on separation of sheets 12 and 22. Release layer 16 is designed such that its cohesivity and its adhesion to either first adhesive layer 18 or porous or particulate layer 14 is less, in exposed regions, than the adhesion of layer 14 to heat-activated zone or layer 12c. The result of these relationships is that release layer 16 undergoes an adhesive failure in exposed areas at the interface between layers 16 and 18, or at the interface between layers 14 and 16; or, as shown in Figure 2, a cohesive failure of layer 16 occurs, such that portions (16b) are present in image 10b and portions (16a) are adhered in exposed regions to porous or particulate portions 14a. Release layer 16 can comprise a wax, wax-like or resinous material. Microcrystalline waxes, for example, high density polyethylene waxes available as aqueous dispersions, can be used for this purpose. Other suitable materials include Carnauba wax, beeswax, paraffin wax and wax-like materials such as poly(vinyl stearate), poly(ethylene sebacate), sucrose polyesters, polyalkylene oxides and dimethylglycol phthalate. Polymeric or resinous materials such as poly(methyl methacrylate) and copolymers of methyl methacrylate and monomers copolymerizable therewith can be employed. If desired, hydrophilic colloid materials, such as poly(vinyl alcohol), gelatin or hydroxyethyl cellulose can be included as polymer binding agents. Resinous materials, typically coated as latices, can be used and latices of poly(methyl methacrylate) are especially useful. Cohesivity of layer 16 can be controlled to provide the desired and predetermined fracturing. Waxy or resinous layers which are disruptible and can be fractured sharply at interfaces between their particles can be added to the layer to reduce cohesivity. Examples of such particulate materials include silica, clay particles and particles of polytetrafluoroethylene. The imaging medium 10 incorporates first and second adhesive layers 18 and 20, which are as described in U.S. Patent No. 5,275,914 and EP-A-581,144. The first adhesive layer 18 comprises a polymer having acidic groups thereon, preferably carboxyl groups. On contact with the second adhesive layer 20, first adhesive layer 18 serves to develop rapidly substantial pre-curing and post-curing adhesion to the second adhesive layer 20, thus securing the first and second elements together to form the unitary laminar imaging medium 10. A specific preferred copolymer for use in layer 18 is that available asNeocryl BT 520 from ICI Resins (U.S.), Wilmington, Massachusetts 01887-0677, United States of America. This material is an acrylic copolymer containing sufficient free carboxyl groups to permit solubility in water that contains ammonia. The second adhesive layer 20 of imaging medium 10 comprises a hardenable adhesive layer which protects the medium against stresses that would create a delamination of the medium, typically at the interface between zone or layer 12c and image-forming layer 14. The physical stresses which tend to promote delamination but can be alleviated by hardenable layer 20 can vary and include stresses created by bending the laminar medium and stresses created by winding, unwinding, cutting, slitting or stamping operations. Since hardenable layer 20 can vary in composition, it will be appreciated that a particular adhesive may, for example, provide protection of the medium against delamination promoted by bending of the medium, while providing little or no protection against delamination caused, for example, by a slitting or stamping-and- cutting operation, or vice versa. Imaging medium 10 is normally prepared by the lamination of first and second sheet-like web elements or components, the first element or component comprising web material 12 carrying image-forming layer 14, release layer 16 and first adhesive layer 18, while the second element comprises second adhesive layer 20 and second web material 22. The two elements can be laminated under pressure, and optionally under heating conditions, to provide the unitary and laminar thermally actuatable imaging medium 10 of the invention. Upon curing of second adhesive layer 20, medium material 10 is ready for imaging. Attachment of weakly adherent image-forming layer 14 to heat-activatable zone or layer 12c in areas of exposure is accomplished by (a) absorption of radiation within the imaging medium; (b) conversion of the radiation to heat sufficient in intensity to heat activate zone or layer 12c; and (c) cooling to more firmly join exposed regions or portions of layer 14 to heat-activatable zone or layer 12c. Thermal imaging medium 10 can absorb radiation at or near the interface of layer 14 with heat-activatable zone or layer 12c. This is accomplished by using layers in medium 10 which by their nature absorb radiation and generate the requisite heat for desired thermal imaging, or by including, in at least one layer, an agent which can absorb radiation of the wavelength of the exposing source. Thermal imaging medium 10 can be imaged by creating (in medium 10) a thermal pattern according to the information imaged. Exposure sources providing radiation which can be directed on to medium 10, and converted by absorption into thermal energy, can be used. Gas discharge lamps, xenon lamps and lasers are examples of such sources. The exposure of medium 10 to radiation can be progressive or intermittent. For example, a medium as shown in Figure 1 can be fastened on to a rotating drum for exposure of the medium through sheet 12. A radiation spot of high intensity, such as is emitted by a laser, can be used to expose the medium 10 in the direction of rotation of the drum, while the laser is moved slowly in a transverse direction across the web, thus tracing out a helical path. Laser drivers, designed to fire corresponding lasers, can be used to intermittently fire one or more lasers in an imagewise and predetermined manner to record information according to an original to be imaged. As shown in Figure 2, a pattern of intense radiation can be directed on to medium 10 by exposure to a laser from the direction of the arrows 24, 24', 26 and 26', the areas between the respective pairs of arrows defining regions of exposure. If desired, the imaging medium can be imaged using a moving slit, stencils or masks, and by using a tube, or other source, which emits radiation continuously and can be directed progressively or intermittently on to medium 10. Thermographic copying methods can also be used. Preferably, a laser or combination of lasers is used to scan the medium and record information as very fine dots or pels. Semiconductor diode lasers and YAG lasers having power outputs sufficient to stay within upper and lower exposure threshold values of medium 10 will be preferred. Useful lasers may have power outputs in the range of from 40 to 1000 milliwatts. An exposure threshold value, as used herein, refers to a minimal power required to effect an exposure, while a maximum power output refers to a power level tolerable by the medium before "burn out" occurs. Lasers are particularly preferred as exposing sources since medium 10 may be regarded as a threshold-type of film; i.e., it possesses high contrast and, if exposed beyond a certain threshold value, will yield maximum density, whereas no density will be recorded below the threshold value. Especially preferred are lasers which can provide a beam sufficiently fine to provide images having resolution as fine as 4,000 - 10,000 dots per inch (160-400 dots per millimeter). Locally applied heat, developed at or near the interface of image-forming layer 14 and heat-activatable zone or layer 12c can be intense (about 400 °C) and serves to effect imaging in the manner described above. Typically, the laser dwell time on each pixel will be less than one millisecond, and the temperature in exposed regions can be between 100°C and 1000°C. Apparatus and methodology for forming images from thermally actuatable media such as the medium 10 are described in detail in U.S. Patent No. 5,170,261 (and the corresponding International Application No. PCT/US91/06880, Publication No. WO 92/10053); and in U.S. Patent No. 5,221,971 (and the corresponding International Application No. PCT/US91/06892, Publication No. WO 92/10057). The imagewise exposure of medium 10 to radiation creates in the medium latent images which can be viewed upon separation of the sheets 12 and 22 as shown in Figure 2. Sheet 22 can comprise any of a variety of plastic materials transmissive of actinic radiation used for the photohardening of photohardenable adhesive layer 20. A transparent polyester (e.g., poly(ethylene terephthalate)) sheet material is preferred. In addition, sheet 22 will preferably be subcoated, or may be corona treated, to promote the adhesion thereto of photohardened layer 20. Preferably, each of sheets 12 and 22 will be flexible polymeric sheets. The medium 10 is especially suited to the production of high density images as image 10b, shown in Figure 2. As previously noted, separation of sheets 12 and 22 without exposure, i.e., in an unprinted state, provides a totally dense image in colorant material on sheet 22 (image 10b). The making of a copy entails the use of radiation to cause the image-forming colorant material to be firmly attached to web 12. Then, when sheets 12 and 22 are separated, the exposed regions will adhere to web 12 while unexposed regions will be carried to sheet 22 and provide the desired high density image 10b. Since the high density image provided on sheet 22 is the result of "writing" on sheet 12 with a laser to firmly anchor to sheet 12 (and prevent removal to sheet 22) those portions of the colorant material which are unwanted in image 10b, it will be seen that the amount of laser actuation required to produce a high density image can be kept to a minimum. Since image 10b, because of its informational content, aesthetics or otherwise, will often be considered the principal image of the pair of images formed from medium 10, it may be desired that the thickness of sheet 22 be considerably greater, and the sheet 22 thus more durable, than sheet 12. In addition, it will normally be beneficial from the standpoints of exposure and energy requirements that sheet 12, through which exposure is effected, be thinner than sheet 22. Asymmetry in sheet thickness may increase the tendency of the medium material to delaminate during manufacturing or handling operations. Utilization of photohardenable adhesive layer 20 will be preferred in medium 10 particularly to prevent delamination during manufacture of the medium. In the description of the protective process of the invention given below with reference to Figures 3-6, it will be assumed that it is the image 10b which is to be protected, but no significant changes in the procedure are required to use the same process for the protection of the image 10a. Figure 3 of the accompanying drawings shows in section a laminating sheet (generally designated 30) disposed over the binary image 10b formed on sheet 22, as described above. The laminating sheet 30 comprises an adhesive layer 32, a barrier layer 34, a durable layer 36, a release layer 38 and a support layer 40. The laminating sheet 30 is larger in both footprint dimensions (i.e., length and width) than the sheet 22. Either or both of the adhesive layer 32 and the release layer 38 can be omitted from the laminating sheet in some cases. Some barrier layers can function as their own adhesives without the need for a separate adhesive layer, and some durable layers will release cleanly from the support layer without the need for a separate release layer. As shown in Figure 4, the laminating sheet 30 is laminated to the image 10b so that the adhesive layer 32 adheres to both the first and second areas of the image, and so that the laminating sheet 30 protrudes beyond the periphery of the sheet 22 all around the sheet. Next, the laminating sheet 30 is separated from the image 10b, as shown in Figure 5; conveniently, one edge of the laminating sheet is gripped, manually by an operator or mechanically, and the laminating sheet 30 simply peeled away from the image 10b. As seen in Figure 5, in peripheral portions of the laminating sheet where the adhesive layer 32 is not attached to the image 10b, the peripheral portions 32a, 34a and 36a of the adhesive layer 32, the barrier layer 34 and the durable layer 36 respectively remain attached to the release layer 38 and the support layer 40, while the central portions 32b, 34b and 36b of the adhesive layer 32, the barrier layer 34 and the durable layer 36 respectively remain attached to the image 10b, so that the adhesive layer 32, the barrier layer 34 and the durable layer 36 break substantially along the periphery of the sheet 22, thus providing clean edges on the protected image 10b. Depending upon the nature of the release layer 38, none, part or all of the release layer 38 may remain with the central portions 32b, 34b and 36b of the adhesive layer 32, the barrier layer and the durable layer 36 on the image 10b. The central portions 32b, 34b and 36b of the adhesive layer 32, the barrier layer 34 and the durable layer 36 respectively (with any release layer 38 remaining thereon) form a durable coating over the image 10b, as shown in Figure 6. Figure 7 shows an apparatus 40 which may be used to carry out the lamination process of Figures 3 to 6. This apparatus 40 comprises a feed roll 42 on which is wrapped a supply of laminating sheet 30 (which is shown for simplicity in Figure 7 as comprising only the durable layer 36 and the support layer 40, although it will of course include the barrier layer 34 and other layers as described above), a first guide bar 44 and a pair of electrically heated rollers 46 and 48 having a nip 50 therebetween. The rollers 46 and 48 are provided with control means (not shown) for controlling the temperature of the rollers and the force with which they are driven toward one another, and thus the pressure exerted in the nip 50. The apparatus 40 further comprises a series of second guide bars 52 and a take-up roll 54. Laminating sheet 30 is fed from the feed roll 42, around the guide bar 44 and into the nip 50 under a tension controllable by tension control means (not shown) provided on the feed roll 42 and/or the take-up roll 54. The image 56 to be protected is fed (manually or mechanically), image side up, into the nip 50 below the laminating sheet 30; the laminating sheet is made wider than the image so that excess laminating sheet extends beyond both side edges of the image 56. The heat and pressure within the nip 50 laminate the image 56 to the laminating sheet 30 and the two travel together beneath the guide bars 52, until the laminating sheet is bent sharply around the last of the guide bars 52. Because the thin laminating sheet 30 is more flexible than the image 56, this sharp bending of the laminating sheet causes, in the area where the laminating sheet 30 overlies the image 56, separation of the durable layer 36 from the support layer 40, with the durable layer 36 remaining attached to the image 56, whereas in areas where the laminating sheet 30 does not overlie the image 56, the durable layer 36 remains attached to the support layer 40. The support layer 40, and the areas of the durable layer 36 remaining attached thereto are wound on to the take-up roll 54. The present invention provides protected differential adhesion images, which are resistant to abrasion and solvents, which are suitable for use in exposing second generation images, which can withstand repeated application and removal of graphic arts tape, and which are thus well suited for use in the graphic arts industry. The following Example is now given, though by way of illustration only, to show details of particularly preferred reagents, conditions and techniques used in the process of the present invention. All parts, ratios and proportions, except where otherwise indicated, are by weight. EXAMPLE On to a first sheet of poly(ethylene terephthalate) of 1.75 mil (44 μm) thickness (ICI Type 3284 film, available from ICI Americas, Inc., Hopewell, Virginia, United States of America) were deposited the following layers in succession: a 2.4 μm thick stress-absorbing layer of polyurethane (a mixture of 90% ICI Neotac R-9619 and 10% ICI NeoRez R-9637, both from ICI Resins (U.S.), Wilmington, Massachusetts, United States of America); a 1.3 μm thick heat-activatable layer of poly(styrene-co-acrylonitrile); a 1 μm thick layer of carbon black pigment, poly(vinyl alcohol) (PNA), 1,4-butanediol diglycidyl ether, and a fluorochemical surfactant (FC-171, available from Minnesota Mining and Manufacturing Corporation, St. Paul, Minnesota 55144-1000, United States of America) at ratios, respectively, of 5:1:0.18/0.005; a 0.6 μm thick release layer comprising polytetrafluoroethylene, silica and hydroxyethylcellulose (Νatrosol +330, available from Aqualon Incorporated, Bath, Pennsylvania 18014, United States of America), at ratios, respectively, of 0.5: 1:0.1; and a 2.2 μm thick layer of the aforementioned Νeocryl BT 520 copolymer containing acidic groups. To form the second adhesive layer, 5 parts of butyl acrylate, 82 parts of butyl methacrylate and 13 parts by weight of Ν,Ν-dimethylaminoethyl acrylate were copolymerized with AIBN to form a copolymer having a number average molecular weight of about 40,000 and a glass transition temperature of +11 °C. A coating solution was prepared comprising 11.90 parts of this copolymer, 2.82 parts of trimethylolpropane triacrylate (TMPTA, available as Ageflex TMPTA from CPS Chemical Company, Old Bridge, New Jersey 08857, United States of America), 0.007 parts of 4-methoxyphenol (a free radical inhibitor), 1.14 parts of 2,2-dimethoxy-2-phenylacetophenone (a photoinitiator, available as Irgacure 651 from Ciba-Geigy Corporation), 0.037 parts of tetrakis { methylene(3 , 5-di-tert-butyl-4-hydroxyhydrocinnamate) } methane (an anti- oxidant, available as Irganox 1010 from Ciba-Geigy Corporation), 0.037 parts of thiodiethylene bis(3,5-di-tert-butyl-4 hydroxy)hydrocinnamate (an anti-oxidant, available as Irganox 1035 from Ciba-Geigy Corporation), and 58.28 parts of ethyl acetate solvent. This coating solution was coated on to 4 mil (101 μm) poly(ethylene terephthalate) film (ICI Type 526 anti-static treated film, available from ICI Americas, Inc., Hopewell, Virginia, United States of America; this film forms the second web 22 of the imaging medium 10) and dried in an oven at 85°C (185°F) to a coating weight of 9400 mg/m 2 to form a hardenable second adhesive layer 20 approximately 10 μm thick. The first and second poly(ethylene terephthalate) sheets were immediately brought together with their adhesive layers in face-to-face contact, the 4 mil sheet being in contact with a rotating steel drum. A rubber roll having a Durometer hardness of 70- 80 was pressed against the 1.75 mil sheet. The resulting web of laminar medium was then passed in line, approximately 30 seconds after lamination, under a radio-frequency- powered source of ultraviolet radiation, with the 4 mil sheet facing, and at a distance of 2.5 inches (6.4 cm) from, the source (a Model DRS-111 Deco Ray Conveyorized Ultraviolet Curing System, sold by Fusion UV Curing Systems, 7600 Standish Place, Rockville, Maryland 20855-2798, United States of America), which served to cure the second adhesive layer 20. After curing, the web of imaging medium was passed through a slitting station where edgewise trimming along both edges of the medium was performed in the machine direction. The resultant trimmed web was then wound on to a take-up roll. Individual sheets of imaging medium cut from the resultant roll were imaged by laser exposure through the 1.75 mil sheet using high intensity semiconductor lasers. In each case, the medium was fixed (clamped) to a rotary drum with the 4 mil sheet facing the drum. Radiation from semiconductor lasers was directed imagewise through the 1.75 mil sheet in response to a digital representation of an original image to be recorded in the medium. After exposure to the high-intensity radiation (by scanning of the imaging medium orthogonally to the direction of drum rotation) and removal of the exposed imaging medium from the drum, the two sheets of the imaging medium were separated to provide a first image on the first, 1.75 mil sheet and a second (and complementary) image on the second, 4 mil sheet (the principal image). A first laminating sheet (hereinafter "Sheet A") was prepared having as its support layer a sheet of 0.92 mil (23 μm) smooth poly(ethylene terephthalate). On to this support layer were coated successively: a release layer of polymeric wax; a durable layer; a barrier layer; and an adhesive layer. The fluid used for coating the durable layer comprised a methacrylate polymer together with a thermally activated polymerization initiator. This fluid was coated at from 8 to 15% solids solution, preferably 10% solids solution, to give a coverage of 1.6 ± 20% dried coverage. Drying of the coating was effected in a 30 foot (9.1 m) oven with a web speed of 300 ft/min (91 m/min), the oven being maintained at approximately 250°F (122°C), with the web and coating reaching temperatures of 220- 250°F (103-122°C), sufficient to initiate thermal curing of the layer. The fluid used for coating the barrier layer comprised the aforementioned Daran SL-158, supplied by Hampshire Chemical Corporation. This aqueous fluid was coated at 27 percent solids solution, to give a dried coverage of 0.7 μm. Drying of the barrier layer was effected at 180-240°F (83-116°C) for approximately 25 seconds. The fluid used for coating the adhesive layer comprised Morton Adcote 9636/37 hot melt adhesive, sold by Morton International, Inc., 3334 West Wacker Drive, Chicago, IL 60606, United States of America, coated to a dried thickness of about 1.5 μm. A second laminating sheet (hereinafter "Sheet B") was prepared in the same manner except that the barrier layer was 2.5 μm thick. To provide a control, a third laminating sheet (hereinafter "Sheet C") was prepared in the same manner except that the barrier layer was omitted. Each laminating sheet was separately laminated on a laminator having a roller durometry of from 55 to 70 Shore A, a hot roller temperature of 185°F (85°C), a piston air pressure of 90 psig (0.74 MPa) and a speed setting of 5 feet/minute (1.52 m/min) to a black halftone image prepared as described above. After each lamination, the laminating sheet was peeled from the image, causing a failure to occur in the wax release layer and leaving a glossy surface of wax, durable layer, barrier layer (except with control Sheet C) and adhesive layer on the image. To test the solvent resistance of the protected images thus prepared, each of six commercial graphic arts cleaning solvents was applied to a cotton wipe and manually rubbed 50 times (i.e., 25 strokes in each direction) under a pressure of 4-5 pounds (1.8-2.3 kg) over a portion of the protected image. The protected image was deemed to have past the test if, after the solvent rubbing, there was no visible change in the appearance of the protected image. The solvents used in these tests were as follows: Anchor 1, sold by Anchor Lithkemko, 50 Industrial Loop North, Orange Park, FL 32073, United States of America; analysis indicated this material comprised 5- 15 percent isopropanol and 85-95 percent hexane; Hurst 150, sold by Hurst Graphics, Inc., 2500 San Fernando Road, Los Angeles CA 90065, United States of America; analysis indicated this material comprised 0.5-1.5 percent cyclohexane and 5-9 percent toluene, with the balance being heptane and methylcyclohexane; Varn, sold by Varn Products, 905 South Westwood, Addison JX 60101, United States of America; analysis indicated this material comprised 10 percent isopropanol and 90 percent hexane; Hawson, sold by E. I. Du Pont de Nemours & Co., Wilmington DE 19898, United States of America; analysis indicated this material comprised 50 percent hexane and 50 percent heptane; Sprayway #205, sold by Sprayway, Inc., 484 Vista Avenue, Addison IL 60101, United States of America; analysis indicated this material comprised 1-5 percent of carbon dioxide and about 97 percent of trichlorotrifluoroethane; and #1 Network, sold by #1 Network, Inc. P.O. Box 24807, Jacksonville FL 32241, United States of America; analysis indicated this material comprised 90-95 percent of 1,1,1-trichloroethane, together with small amounts of carbon dioxide, dimethoxymethane and 2-methyl-2-proρanol. The results are shown in the Table below. Table Sheet A Sheet B Sheet C Anchor 1 Pass Pass Fail Hurst 150 Pass Pass Fail Varn Pass Pass Fail Hawson Pass Pass Fail Sprayway # 205 Pass Pass Fail #1 Network Pass Pass Fail From the data in this Table, it will be seen that the barrier layer was effective in improving the solvent resistance of the protected images, even at the 0.7 μm barrier layer thickness in Sheet A.

Claims: CLAIMS 1. A process for protecting a binary image, this binary image comprising a plurality of first areas, at which a porous or particulate image-forming substance is adhered to a substrate, and a plurality of second areas, at which the substrate is free from the image-forming substance, which process comprises: providing a laminating sheet comprising a durable layer and a support layer, the durable layer being substantially transparent ; laminating the laminating sheet to the binary image so that the durable layer adheres to both the first and second areas of the image; and separating the support layer from the image such that the durable layer remain attached to the image, thereby covering the image with a durable layer characterized in that the laminating sheet further comprises a barrier layer disposed on the opposed side of the durable layer from the support layer, this barrier layer being substantially transparent and comprising a polymeric organic material substantially impervious to the passage of hexane, isopropanol and water therethrough, such that, following the lamination, the barrier layer adheres to both the first and second areas of the image, and after the separation of the support layer from the image the barrier layer remains attached to the image. 2. A process according to claim 1. characterized in that the laminating sheet further comprises a release layer interposed between the durable layer and the support layer, such that, in the areas where the barrier and durable layers remain attached to the image, separation of the durable layer from the support layer occurs by failure within or on one surface of the release layer. 3. A process according to either of the preceding claims characterized in that the barrier layer comprises polymerized repeating vinylidene chloride units. 4. A process according to claim 3 characterized in that the barrier layer comprises copolymerized repeating units from an ethylenically unsaturated monomer copolymerizable with vinylidene chloride. 5. A process according to claim 4 characterized in that the ethylenically unsaturated monomer is an acrylate or methacrylate ester. 6. A process according to any one of the preceding claims characterized in that the barrier layer comprises a polyurethane. 7. A process any one of the preceding claims characterized in that the durable layer comprises a siloxane, the siloxane being incorporated into a polymeric material in such a manner that it is not removed therefrom be hexane, isopropanol or water. 8. A process any one of the preceding claims characterized in that the binary image is formed by: providing a layer of a porous or particulate image-forming substance on a heat-activatable image-forming surface of a substrate, the layer of the image-forming substance having a cohesive strength greater than the adhesive strength between the layer and the substrate, thereby providing a thermal imaging medium; imagewise subjecting portions of the thermal imaging medium to exposure to brief and intense radiation, thereby firmly attaching exposed portions of the image-forming substance to the substrate; and removing from the substrate those portions of the image-forming substance not exposed to the radiation. 9. A protected binary image, the image comprising a plurality of first areas at which a porous or particulate image-forming substance is adhered to a substrate and a plurality of second areas at which the substrate is free from the image-forming substance, and a substantially transparent durable layer covering the image and adhered to both the first and second areas of the image, the protected image being characterized by a barrier layer also covering the image and disposed between the barrier layer and the image, the barrier layer being substantially transparent and adhered to both the first and second areas of the image, the barrier layer comprising a polymeric organic material substantially impervious to the passage of hexane, isopropanol and water therethrough. 10. A protected binary image according to claim 9 characterized in that the durable layer comprises a siloxane, the siloxane being incorporated into a polymeric material in such a manner that it is not removed therefrom be hexane, isopropanol or water. 11. A protected binary image according to claim 9 or 10 characterized in that the barrier layer comprises polymerized repeating vinylidene chloride units. 12. A protected binary image according to claim 11 characterized in that the barrier layer comprises copolymerized repeating units from an ethylenically unsaturated monomer copolymerizable with vinylidene chloride. 13. A protected binary image according to claim 15. wherein the barrier layer comprises a polyurethane.

Inventors: CONFORTI ROBERT M, KIM SUN-WOOK, YAO BEING-KUNG, CONFORTI, ROBERT, M., KIM, SUN-WOOK, YAO, BEING-KUNG
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1994
Main classification: B41M5
IPC codes: B41M5, B41M7